A conventional hydraulic chuck disclosed in Taiwanese Patent No. 557239 includes a chuck body, a fixed seat fixedly mounted in the chuck body, a rotary seat rotatably mounted in the fixed seat, a driving seat moveably mounted to the rotary seat, a front jaw unit, a rear jaw unit, and an oil path that passes through the chuck body, the fixed seat and the rotary seat. When pressurized oil flows through the oil path to move the driving seat, the driving seat drives the front and rear jaw units to grip or to release a workpiece.
A gap through which the pressurized oil flows is formed between an outer surface of the rotary seat and an inner surface of the fixed seat, and a plurality of seal rings may be disposed between the outer surface of the rotary seat and the inner surface of the fixed seat to prevent leakage of the pressurized oil. When the pressurized oil flows through the gap to adjust the front and rear jaw units, the seal rings are deformed by the pressurized oil so as to seal up the gap to prevent leakage of the pressurized oil. When the pressurized oil is prevented from flowing through the gap after the adjustment of the front and rear jaw units, the seal rings are restored to permit relative rotation between the rotary seat and the fixed seat.
However, each of the seal rings may have a circular cross-section, and may not be sufficiently deformed enough to seal up the gap when the pressurized oil flows through. If the pressure of the oil is increased in order to increase the degree of deformation of the seal rings, more serious leakage of the pressurized oil would have occurred.
Moreover, if the seal rings are designed to constantly seal up the gap even when the rotary seat rotates relative to the fixed seat, the seal rings may be easily worn out, and may be deteriorated due to the high-temperature resulted from the frictional contact among the seal rings and the rotary seat.